1. Field of the Invention
The present invention relates to a light-emitting diode element composed of a nitride semiconductor.
2. Description of the Related Art
Recently, a semiconductor element composed of a nitride semiconductor such as gallium nitride (GaN) has been researched and developed actively. A semiconductor light-emitting element composed of nitride semiconductors comprising aluminum nitride (AlN), gallium nitride (GaN) or, indium nitride (InN), or mixed crystal thereof emits light in a wide wavelength region from ultraviolet or blue to infrared by varying its film composition. As its application, a visible-range light-emitting diode using a nitride semiconductor has already been commercially-available (for example, Shuji Nakamura et. al., Jpn. J. Appl. Phys. Vol. 34 (1995) L. 1332-L.1335).
FIG. 13 shows a cross-sectional view of a light-emitting diode element according to prior art. As shown in FIG. 13, the conventional light-emitting diode element has a low-temperature grown buffer layer 102 consisting of GaN, an n-type GaN cladding layer 103, a quantum well layer 104 consisting of InxGa1-xN, a p-type cladding layer 105 consisting of p-type GaN, and a p-type contact layer 106 consisting of p-type GaN. These layers are grown in this order on the main surface of a sapphire substrate 101 with a plane orientation of (0001). A p-side electrode 107 is formed on the p-type contact layer 106, and an n-side electrode 108 is formed on the selectively exposed region of the n-type GaN layer 103.
According to the prior art, in order to suppress carrier recombination due to non-luminescence transition derived from lattice defects or threading dislocations, a nitride semiconductor film having significantly low defects in the crystal has to be prepared. For this reason, a monocrystalline substrate such as a sapphire substrate is used, but such a monocrystalline substrate is very expensive.
In order to solve the above-mentioned problem, a method for preparing a polycrystalline nitride semiconductor film on a graphite substrate by a pulse sputtering method has been proposed (Japanese Laid-open patent publication No. 2009-200207).
However, since the GaN film prepared on the graphite substrate by a pulse sputtering method is polycrystalline and has many defects, it fails to be suitable for light-emitting diodes or electronic devices. Furthermore, since the nitride semiconductor film prepared by a sputtering method is damaged severely due to discharged plasma during the film growth, it is well known that the crystal of the film contains extremely many defects. Accordingly, in a case where the method for preparing the nitride semiconductor by the pulse sputtering method according to Patent Document 1 is used, it is very difficult to prepare p-type GaN necessary to prepare a light-emitting diode element.
The purpose of the present invention is to provide a nitride light-emitting diode element with high performance at low cost with use of a graphite substrate, which is non-monocrystalline substrate, by a Metal Organic Chemical Vapor Deposition (MOCVD), which is most suitable for fabricating a semiconductor device.